A hybrid electric vehicle (HEV) or electric vehicle, referring to FIG. 1, includes a high-voltage battery 10. An inverter 12 inverts a DC voltage into an AC voltage to supply the inverted AC voltage to a driving motor. An auxiliary battery 14 supplies the DC voltage to electric components of the vehicle when the vehicle starts and provides the voltage required to operate a main relay of the high-voltage battery 10. A low-voltage DC/DC converter (LDC) converts the DC voltage supplied from the high-voltage battery 10 to charge the converted voltage in the auxiliary battery 14 or supplies the DC voltage to the electric components.
The LDC 16 converts the voltage of the high-voltage battery 10 into a low voltage (+12V) to charge the auxiliary battery 14 like an alternator of a combustion engine. The LDC 16 supplies the converted voltage to various kinds of electric components (electric loads).
The LDC operates only when a starting (HEV ready) condition of the HEV is satisfied to charge the auxiliary battery or to supply the voltage to the electric components (including various kinds of controllers).
A conventional control method of operating the LDC when the HEV starts will be described as follows.
As shown in a flowchart of FIG. 2, the HEV is in a starting ready (HEV ready) state in which the HEV is capable of starting when steps are satisfied. Here, the steps include trying ignition start (IG start) by a driver under a condition in which the charged voltage of the auxiliary battery is normal, performing an immobilizer authentication process for vehicle security, turning on the main relay of the high-voltage battery, confirming that the electric loads including various kinds of controllers can be operated by the voltage supplied from the auxiliary battery, and so on.
In this case, the LDC becomes operable after the HEV starts to charge the auxiliary batter with the voltage from the high-voltage battery or to supply the voltage to various kinds of electric components.
However, in a case where the voltage of the auxiliary battery is lowered to a reference voltage (normal voltage) or less due to a low-temperature condition, such as in the winter, deterioration of durability caused by a superannuation of the vehicle, etc., the HEV may not start.
Referring to FIG. 3, in a case where the voltage of the auxiliary battery is reduced to a normal level (about 9V or more), operating voltage of various kinds of controllers related to the starting, such as a battery management system (BMS) and a motor controller (MC), is higher than the voltage of the auxiliary battery, which is reduced to the normal level or less, even though the step of ignition on (IG ON), the step of turning on the main relay, and the step of trying the ignition start (IG start) are performed. As a result, since starting of the HEV is impossible, the LDC does not operate.
That is, the auxiliary battery does not supply a suitable voltage for the operating voltage of each controller when the voltage of the auxiliary battery is lowered to the normal level or less, and therefore, the starting of the HEV is impossible.